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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let age = 18
        let name = "Jack"
        let flag = false
        let buy
        console.log(age)
        console.log(name)
        console.log(flag)
        console.log(buy)
        console.log(typeof age)
        console.log(typeof name)
        console.log(typeof flag)
        console.log(typeof buy)
    </script>
</body>
</html>